The drilling of wells, for example, for oil and gas production, conventionally employs relatively small diameter strings of drill pipe to which is secured a drill bit of somewhat larger diameter. After a selected portion of the well bore has been drilled, the wellbore is usually lined with a string of tubulars known as casing. The term casing is used herein to encompass any wellbore liner. The casing normally has a larger diameter than the drill pipe and a smaller diameter than the operational drill bit. This conventional system which requires sequentially drilling the borehole using drill pipe with a drill bit attached thereto, pulling the drill pipe out of the hole and running casing into the borehole is time consuming and costly. In addition, each time that a drilling bit needs to be changed, which happens several times during any drilling operation, the drill pipe must be tripped in and out. As a consequence, the process of drilling with casing is gaining popularity as a method of drilling wherein the casing is used as the drilling conduit though which the bit is moved, and after drilling, the casing remains downhole to act as the wellbore liner.
To achieve simultaneous drilling and casing, a specialized drilling assembly is required which drills a borehole of sufficient diameter to accommodate the casing and which is retrievable through the casing. The drilling assembly typically includes a drill bit and one or more hole enlargement tools such as for example an underreamer. The drilling assembly is deployed on the advancing end of the casing. The drill bit can be retractable and/or removable through the casing by electric wireline, braided wire rope or other means.
When a drilling operation is complete the drill bit is retracted through the casing and the casing is left downhole for lining the well. Completion of the cased well, which requires pumping cement into the annulus between the casing and the wellbore wall, is difficult in wells formed using casing drilling since the casing does not contain a cement float shoe, also known as a cement float collar. Since it is necessary to complete a wellbore with cement, the cement was pumped down through the casing and maintained in the annulus by holding a pressure within the casing until the cement hardens.
While previous through-tubing cement float collars are known such as those described in U.S. Pat. Nos. 4,413,682, 5,323,858, 3,159,219 and 4,589,495, those float collars and methods for completion are not useful in casing drilling operations. In particular, a casing string having inner restrictions for latching a through tubing float collar is not suitable for use in casing drilling. The manipulation of the casing string or cement float collar using a tubing string within the casing is not suitable for most casing drilling operations.